(1) Field of the Invention
This invention relates to the chemical treatment of cellulosic textiles to improve the physical properties. More specifically, this invention pertains to an improved formulation that imparts to cellulosic textiles high levels of resilience in combination with high levels of abrasion resistance, breaking strength, and tearing strength. (2) It is well known to impart durable wrinkle resistance to cellulosic fabrics such as cotton, by impregnation with an aqueous solution of a suitable thermosetting resinous precondensate or cellulose crosslinking agent, usually with appropriate catalyst, and eventually curing the impregnated fabric. Such treatments have been effective in improving the wrinkle resistance and shape-holding properties of cotton fabrics and have resulted in greatly increased demand for "easy-care", "wash-and-wear", and "permanent-press" cotton fabrics because they combine traditional comfort, washability, and economy of native fibers with easy care properties that are desired in today's textile market.
A variety of processes has been developed and used for improving wrinkle resistance or wrinkle recovery of fabrics and garments. These processes are known in general as pad-dry-cure resin treatments, where one or more resins are applied to the fabrics through padding, and the fabrics are partially dried before the resin is cured.
The conventional thermosetting resin systems, either postcured or precured, result in embrittlement and reduction of mobility of the microstructural units of cellulosic fibers to such an extent that abrasion resistance, breaking strength, and tearing strength are seriously impaired. Abrasion resistance is often reduced by 75-80%, breaking strength by 50-60%, and tearing strength about by about 50%.
Over the last several years considerable research has been conducted to find ways of overcoming this problem without compromising the wash-wear or durable-press performance of the fabric. Many variations from the pad-dry-cure process have been developed in attempts to solve the problem. These include processes involving a multistage padding and curing, processes involving a pad and wet-fixation prior to cure, and processes involving polymeric additives. The results achieved through all of these porcesses have been marginal and the processes have often been found to be cumbersome and expensive.
The particularly pertinent prior art relative to this invention starts with U.S. Pat. No. 3,125,405, which includes a summary of the foregoing art. This patent describes a process wherein N-methylolacrylamide is reacted in cotton by vinyl polymerization (i.e., at the H.sub.2 --C.dbd.CH--) and is also reacted with the hydroxyl group of the cellulose molecule (i.e., etherification reaction between the --CH.sub.2 OH and the hydroxyl groups of cellulose). This is accomplished by including in the reagent solution a free radical initiator for the vinyl polymerization; this initiator may be acidic in character or acid-generating under the conditions of the curing reaction with cotton, or it can be supplemented with an acid catalyst. In the single-step reaction the N-methylolacrylamide penetrates the fibers, yarns, and fabric and polymerizes to provide long-chain molecules which become attached to cellulosic molecules by means of ether linkages resulting from the reaction of methylol groups in the amide monomer or polymer with the hydroxyl groups of the cellulose. In two alternative processes, the N-methylolacrylamide may be polymerized by vinyl addition with a free-radical initiator before or after reaction is catalyzed by acid between the methylol groups and cellulosic hydroxyl groups. The one-step operation is more efficient for a given amount of reagent, and is preferred for this reason; a one-step operation is substantially more economical and much preferred for textile mill operation. Whether the reaction is conducted in one or two steps, the curing reactions at elevated temperature are conducted at 300.degree. F. for periods of a few minutes. The preferred acid-generating initiators for vinyl polimerization are ammonium and alkali metal persulfates. Acid catalysts are employed together with these persulfate catalysts, or in a separate step there is included a treatment with bisulfate, ammonium chloride, amine salts of hydrochloric or sulfuric acid, or a variety of organic acids.
In U.S. Pat. No. 3,125,406, improved products are indicated to result from the reaction of N-methylolacrylamide with cotton when the reaction is conducted in a special two-stage manner. The preliminary stage involves catalysis with a persulfate catalyst together with an amine-hydrochloride (acid catalyst) and the curing reaction at elevated temperature (90.degree.-175.degree. C.) is carried no further than to reduce the moisture content of the fabric to at least 10% by weight, based on the weight of the dry fabric. This partially reacted fabric is rinsed to remove unreacted N-methylolacrylamide and catalyst residues; the fabric is activated with an acid-generating catalyst (i.e., zinc nitrate, zinc chloride, sodium nitrate, potassium nitrate), which may be supplemented with a salt having high solubility in water, such as sodium or potassium nitrate or calcium bromide. At this point, the fabric is subjected to a second curing reacting at an elevated temperature similar to that of the first cure. The presence of the latter salt is said to enhance the creaseproofing effect for a given amount of acid-generating catalyst. This invention is said to provide products of improved character in that they have better strength, show less discoloration, and are subject to little or no damage as a result of treatments using conventional chlorine or hypochloride bleaching agents and subsequent drying or ironing.
In U.S. Pat. No. 3,246,946 the same improvements in product are said to result from a modification of the process described in U.S. Pat. No. 3,125,405, in which the N-methylolacrylamide is supplemented with limited proportions of N-methylol reagents such as those derived from urea, ethyleneurea, melamine and related compounds. The supplementary N-methylol reagent may be introduced into reaction systems such as those described in U.S. Pat. No. 3,125,405. However, the one-step operation is said to be preferred and generally more efficient for a given amount of reagent.
An improvement in removal of the soiling from the carbon black is claimed in U.S. Pat. No. 3,322,569 wherein cotton fabric is treated with an aqueous solution of a conventional dimethylol reagent (e.g., dimethylolethyleneurea) and a copolymer or butyl acrylate with N-methylolacrylamide, together with low concentrations of N-methylolacrylamide, and a compound containing two polymerizable double bonds such as divinyl benzene or glycol diacrylate.
Three additional patents deal with the reaction of N-methylolacrylamide with cotton fabrics. They are mentioned here to complete the picture, although they are not as pertinent as the foregoing patents.
U.S. Pat. No. 3,558,263 describes a method of inhibiting crosslinking of cellulose when N-methylolacrylamide is reacted with cellulosic textile materials in the presence of an acidic or acid-generating catalyst, e.g., zinc nitrate. In this case potassium iodide (0.1-1.0%) is introduced into the system with or without cupric chloride (ca. 0.1%). In U.S. Pat. No. 3,561,916 a single-step reaction between N-methylolacrylamide and cotton fabric is catalyzed with zinc nitrate to obtain a cotton fabric having high levels of dry wrinkle recovery with low strength loss due to chlorine retention. The process of application of the reagent plus catalyst to fabric in this case is generally similar to that described in U.S. Pat. No. 3,125,405, even though the zinc nitrate catalyst is not known to cause polymerization of the vinyl monomer. In U.S. Pat. No. 3,669,609 citric acid or dicyanamide is added to N-methylolacrylamide to cause a substantial reduction in the yellowing of cellulosic fibers that are treated with this reagent.
The foregoing Prior Art patents fall into one of these categories:
(a) Those that deal with a process or product improvement other than that involving high resilience (durable-press rating and conditioned wrinkle recovery angle) with high retentions of abrasion resistance and strength, or
(b) That patent (U.S. Pat. No. 3,125,405) which deals with the combination of high levels of resilience with abrasion resistance and strength, for which results are achieved in a one-step process and for which experimental data will be illustrated in the Examples of the process of the present invention (below).